The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a system and method to correct magnetic gradient non-uniformities that occur in phase contrast MR images used in reconstructing quantitative flow analysis images. The present, invention can also be applied to correct maps of diffusion coefficients measured with diffusion weighted imaging.
Quantitative phase contrast imaging is a growing application. In one embodiment, a phase contrast scan is acquired such that the imaging plane is approximately perpendicular to the vessel of interest. The phase contrast image contains quantitative velocity (cm/s) information. Flow (ml/min) information can be obtained by summing the product of the perpendicular velocity component and the area of each pixel over the cross section of the vessel. Hereinafter, this procedure will be referenced to as "flow analysis".
A systematic error in the flow analysis procedure is the actual gradient strength of the applied imaging and velocity encoding gradients and can differ from the nominal strength at some locations within the bore of the magnet. In standard, non-phase contrast imaging this gradient non-uniformity is corrected with a procedure called "GradWarp", which is described in detail in commonly assigned U.S. Pat. No. 4,591,789 issued May 27, 1986 to Glover et al, and is applied as described in commonly assigned U.S. Pat. No. 5,642,047, issued Jun. 24, 1997 to Bernstein, both of which are incorporated herein by reference. When the gradients of a magnetic field vary, or are non-uniform, the resulting images can be distorted, or warped. This problem is typically exasperated with increasing distance from the isocenter of the magnet. These images can be corrected using such a GradWarp geometric correction operation. Such gradient non-uniformities have previously been ignored for the flow encoding gradients. However, these non-uniformities do in fact create inaccuracies in the velocities that are measured in flow analysis. Flow analysis can be done upon any moving matter in the body, and commonly includes obtaining the flow velocities and/or volume flow rates of blood, cerebral spinal fluid, or the velocity of the myocardium. The non-uniformities can cause the measured velocity to differ from the actual velocity in regions far from the magnet isocenter.
It would therefore be desirable to have a system and method to correct for such systematic errors occurring as a result of magnetic gradient non-uniformity in phase contrast MR images for quantitative flow analysis that solves the aforementioned problems.